In the last few years, fluorescent semiconductor nanocrystals (NCs) have developed greatly in terms of control of their size, shape and composition, providing exceptional control over their properties, allowing for their implementation in a variety of applications such as displays.
The NCs are characterized by a wide absorption spectrum accompanied by a narrow and sharp emission spectrum at the band edge, which enable the simultaneous excitation of NCs with different emission wavelengths using the same lighting source. The fluorescent semiconductor NCs also show outstanding optical and chemical stability under light irradiation over long periods of time. In addition, they are easily adapted to specific applications by the ability to design and control the emission color and properties by tuning their size, shape and composition. Their surface chemistry can be adjusted for dispersion in a specific medium, both in organic and polar media, by proper selection of the stabilizing moieties.
One approach used to achieve applicable devices is layer deposition by different printing techniques [1-9], in particular, semiconductor NCs have been recently introduced successfully into flat panel displays, serving as color converters and emitting entities providing liquid crystal displays with exceptional high color gamut and brightness [10].
Inkjet printing is an important wet deposition method for nanoparticles (NPs), which is commonly used in industrial and domestic applications. Previous reports discuss the printing of NPs [11-14]. Inkjet printing of fluorescent semiconductor NCs quantum dots (QDs) was also achieved [15-24].
Although there are many advantages for the use of fluorescent semiconductor QDs for printing applications, their arrangement in proximity on a substrate leads to optical interference due to particle-particle interactions. These interactions may result in Forster resonance energy transfer (FRET) as well as in self-absorption effects. The phenomenon of self-absorption, in which particles absorb the light emitted by other particles, is highly pronounced in QDs, even after a growth of an outer shell on the emission center. This phenomenon is caused by the significant overlap between the absorption and the emission spectra of the QDs, which leads to efficient re-absorption of the emission. The self-absorption effect causes the effective external emission quantum yield (QY) to decrease significantly and induces changes in the fluorescent color by shifting the emission energy to longer wavelengths. A similar degradation in emission properties is also induced by the FRET process by which an excited QD serves as a donor to transfer this excitation through non-radiative dipole-dipole interactions to neighboring QD serving as an acceptor. While the re-absorption effect becomes significant especially in cases of high optical density samples, the FRET interaction takes place in instances in which particles are in close proximity as is often required in thin fluorescent layers.